After decades of research on inertial confinement fusion (ICF), the National Ignition Facility (NIF) was recently completed at the Lawrence Livermore National Laboratory in California. This installation is designed to demonstrate the nuclear fusion of the hydrogen isotopes, deuterium and tritium, known as D-T fuel. If successful, this achievement will be a milestone in the harnessing of nuclear energy for peaceful applications.
The guts of the NIF are 192 laser beams, which are focused on target pellets containing the D-T fuel. When subjected to the incident radiation, a capsule's outer shell ablates, causing the target to implode. Thereby tremendous pressure and temperatures are attained within the capsule-sufficient to initiate hydrogen fusion. The fuel is held together long enough by inertia in order to realize propagation of the burn.
While all the components of the NIF are vital to success, the design of the fuel pellets is of special concern. Deceptively simple in shape, these pellets must nevertheless perform to perfection. Physically they generally comprise a hollow sphere that is filled with the gaseous D-T fuel. The choice of materials for the shell, however, is critical.
The early designs for fuel pellets consisted of tiny glass spheres. They were easy to fabricate, had good diagnostics, and produced valuable data. However, looking for improvement in the coupling of the incident radiation, scientists sought newer materials for the pellets.
Attention then turned to plastics. These polymers were easy to mold and were available in many compositions. Most important, excellent sphericity and a high degree of smoothness could be attained. These capsules were filled with D-T fuel and then subjected to cryogenic temperatures. This cooling caused a thin layer of frozen or liquid D-T fuel to form on the inside surface of the plastic shell. Such a configuration was favored so as to achieve better implosion.
The latest pellet designs are contemplating novel materials for the fabrication of capsules. (Science & Technology Review, July/August 2007, pp. 12-19) One of these capsules has a copper-doped beryllium shell to which is attached a 10 micrometer fill tube to introduce the D-T fuel. The manufacturing process to make the pellets is complex and requires extraordinary care to meet specification. An alternative design to the beryllium capsule uses high-density carbon, which, like beryllium metal, has a low atomic number. Fabrication of these targets will be equally challenging.